It’s because people read directly from email recommendations, and are not logged in to the Medium app. When they want to interact with comments, they are told to switch to the app. They do, but won’t read the story again. They might clap again though. It will be registered as a non-read..
I have received several comments on my stories, where the stats show “zero views" but the person commenting obviously read the story (outside the app) because they are showing that they are familiar with my story’s content.

So this is a reoccurring glitch. Don’t be so sure people are mostly clapping without reading.